Optical system and method for illumination of reflective spatial light modulators in maskless lithography

ABSTRACT

An illuminator for a lithography system is provided. The illuminator includes a mask positioned along an optical axis and first and second refractive groupings positioned along the axis in cooperative arrangement with the mask. Also included are first and second reflecting devices for reflecting an image output from the first and second refractive groupings and a spatial light modulator (SLM) positioned along the axis in cooperative arrangement with the first and second reflecting devices. The active areas of the mask and the SLM are positioned off-axis.

BACKGROUND

1. Field of the Invention

The present invention relates to a lithographic system. Moreparticularly, the present invention relates to concepts of illuminationwithin a maskless lithography system.

2. Related Art

A lithographic system is a machine that applies a desired pattern onto atarget portion of a substrate. The lithographic system can be used, forexample, in the manufacture of integrated circuits (ICs), flat paneldisplays, and other devices involving fine structures. In a conventionallithographic system, a patterning means, which is alternatively referredto as a mask or a reticle, can be used to generate a circuit patterncorresponding to an individual layer of the IC (or other device). Thepattern can be imaged onto a target portion (e.g., comprising part ofone or several dies) on a substrate (e.g., a silicon wafer or glassplate) that has a layer of radiation-sensitive material (e.g., resist).Instead of a mask, the patterning means can comprise an array ofindividually controllable elements that generate the circuit pattern.

In general, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known popular lithographicsystems include steppers & scanners. In steppers, each target portion isirradiated by exposing an entire pattern onto the target portion in asingle pass. In scanners, each target portion is irradiated by scanningthe pattern through the beam in a given direction (the “scanning”direction), while synchronously scanning the substrate parallel oranti-parallel to this direction.

A lithographic system can also be maskless. Maskless lithography, oroptical maskless lithography (OML) as known to those of skill in theart, is an extension of conventional (i.e., mask-based) lithography. InOML, however, instead of using a photomask, millions of micro-mirrorpixels on a micro-electro-mechanical systems (MEMS) device aredynamically actuated in real-time to generate the desired pattern. MEMS,however, are only one class of OML devices. Due to the fixed gridimposed by the pixels and the use of short-pulse duration excimer lasersat deep ultra-violet (DUV) wavelengths, spatial modulation of grayscales is required. This class of MEMS devices are therefore known asspatial light modulators (SLMs).

In conventional maskless lithography systems, unique challenges arepresented with regard to illuminating the SLMs using, for example,excimer lasers. These challenges are that the rays incident on, andreflected off of, the SLM fill the same physical space. This occurrencemakes it difficult to provide spatial separation between the illuminatorand the projection optics (PO) within the system.

One class of maskless lithography systems uses beam splitters to providespatial separation between the illuminator and the POs. Beam splitters,however, are undesirable in the application above because of reasonsdiscussed below. Polarizing beam splitters are generally used to projectthe radiation beam onto the individually controllable elements of theSLM. The radiation beam is projected through the beam splitter twice anda quarter wave plate is used to change the polarization of the radiationbeam after a first transmission through the beam splitter and before asecond transmission through the beam splitter. Use of polarization tocontrol the direction of the radiation beam means that the cross sectionof the radiation beam has a uniform polarization. As a result, differentpolarizations cannot be used to create different effects during theexposure. Also, beam splitters are inefficient and can be difficult tothermally control.

Further still, polarized beam splitters cannot be used, for example, inhigh numerical aperture (NA) maskless lithography because they do notpreserve the polarization state of the light, which is a requirement forhigh NA maskless lithography optical systems. Non-polarized beamsplitters are an alternative. However, non-polarized beam splitters haveunacceptably low transmission. Finally, tilted illuminators cannot beused with high NA PO unless the SLM works in “blazing” mode (non-zerothdiffractive order beams enter the PO after reflection off of SLM). Thisis because of inherent limitations of PO NA for an off-axis pupil.

Therefore, what is needed is a maskless lithography system and methodthat eliminates the need for beam splitters.

SUMMARY

Consistent with the principles of the present invention, as embodied andbroadly described herein, the present invention includes an illuminatorfor a maskless lithography system. The illuminator includes a maskpositioned along an optical axis and first and second refractivegroupings positioned along the axis in cooperative arrangement with themask. Also included are first and second reflecting devices forreflecting an image output from the first and second refractivegroupings and a spatial light modulator (SLM) positioned along the axisin cooperative arrangement with the first and second reflecting devices.The active areas of the mask and the SLM are positioned off-axis.

The lithography system of the present invention includes an illuminatorthat does not include a beam splitter. The elimination of a beamsplitter facilitates preservation of the polarization state.Additionally, in the present invention, the exit pupil of theilluminator is on optical axis of the PO and the illuminator itself hasits own optical axis. As an additional feature, the system of thepresent invention (illuminator and PO) is completely symmetrical (i.e.,the entrance and exit pupils are on same optical axis). This symmetryhelps satisfy the demands of a high NA system and helps preservesillumination state.

Further embodiments, features, and advantages of the present inventions,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIGS. 1 and 2 depict a lithographic apparatus, according to variousembodiments of the present invention;

FIG. 3 depicts a mode of transferring a pattern to a substrate accordingto one embodiment of the invention as shown in FIG. 2;

FIG. 4 depicts an arrangement of optical engines, according to oneembodiment of the present invention;

FIG. 5 is a general illustration of an illuminator arranged inaccordance with the present invention; and

FIG. 6 is an exemplary illuminator arranged in accordance with a firstembodiment of the present invention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers canindicate identical or functionally similar elements.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the following detailed description is notmeant to limit the invention. Rather, the scope of the invention isdefined by the appended claims.

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify the invention. The scope of the invention is not limited tothe disclosed embodiment(s). The invention is defined by the claimsappended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment”, “an embodiment”, “an example embodiment”, etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

It would be apparent to one skilled in the art that the presentinvention, as described below, may be implemented in many differentembodiments of hardware and/or the entities illustrated in the drawings.Thus, the operation and behavior of the present invention will bedescribed with the understanding that modifications and variations ofthe embodiments are possible, given the level of detail presentedherein.

The present invention provides an illuminator and PO constructed withoutthe use of a beam splitter in order to preserve the radiationpolarization state. In the present invention, the illuminator and POhave a common optical axis wherein a set of reflective SLMs are placedoff-axis. This off-axis placement makes it feasible that incidentradiation directed to and reflected off of the SLMs, will be spatiallyseparated. Furthermore, the SLMs in the present invention areilluminated non-telecentrically, i.e. the entrance pupil of PO is atfinite distance from the SLM plane. Aperture stop and pupils are on theoptical axis, which allows achievement of maximum PO NA (limited byrefractive index of medium in the image space or material of the lastelement).

The illuminator also images a mask onto the plane of SLMs. This mask isintended to limit the amount of stray light entering the PO and create a“trapezoidal” irradiance profile at each SLM, which helps resolve imagestitching issues. The mask, unlike the SLM, is usually illuminatedtelecentrically. That is the radiation beams' chief rays are parallel toeach other. Relay optics then create the image of the mask on the SLMplane.

FIG. 1 schematically depicts a lithographic projection apparatus 100according to an embodiment of the present invention. Apparatus 100includes at least a radiation system 102, an array of individuallycontrollable elements 104, an object table 106 (e.g., a substratetable), and a projection system (“lens”) 108.

Radiation system 102 can be used for supplying a beam 110 of radiation(e.g., UV radiation), which in this particular case also comprises aradiation source 112.

An array of individually controllable elements 104 (e.g., a programmablemirror array) can be used for applying a pattern to beam 110. Ingeneral, the position of the array of individually controllable elements104 can be fixed relative to projection system 108. However, in analternative arrangement, an array of individually controllable elements104 can be connected to a positioning device (not shown) for accuratelypositioning it with respect to projection system 108. As here depicted,individually controllable elements 104 are of a reflective type (e.g.,have a reflective array of individually controllable elements).

Object table 106 can be provided with a substrate holder (notspecifically shown) for holding a substrate 114 (e.g., a resist coatedsilicon wafer or glass substrate) and object table 106 can be connectedto a positioning device 116 for accurately positioning substrate 114with respect to projection system 108.

Projection system 108 (e.g., a quartz and/or CaF₂ lens system or acatadioptric system comprising lens elements made from such materials,or a mirror system) can be used for projecting the patterned beamreceived from a beam splitter 118 onto a target portion 120 (e.g., oneor more dies) of substrate 114. Projection system 108 can project animage of the array of individually controllable elements 104 ontosubstrate 114. Alternatively, projection system 108 can project imagesof secondary sources for which the elements of the array of individuallycontrollable elements 104 act as shutters. Projection system 108 canalso comprise a micro lens array (MLA) to form the secondary sources andto project microspots onto substrate 114.

Source 112 (e.g., an excimer laser) can produce a beam of radiation 122.Beam 122 is fed into an illumination system (illuminator) 124, eitherdirectly or after having traversed conditioning device 126, such as abeam expander, for example. Illuminator 124 can comprise an adjustingdevice 128 for setting the outer and/or inner radial extent (commonlyreferred to as σ-outer and σ-inner, respectively) of the intensitydistribution in beam 122. In addition, illuminator 124 will generallyinclude various other components, such as an integrator 130 and acondenser 132. In this way, beam 110 impinging on the array ofindividually controllable elements 104 has a desired uniformity andintensity distribution in its cross section.

It should be noted, with regard to FIG. 1, that source 112 can be withinthe housing of lithographic projection apparatus 100 (as is often thecase when source 112 is a mercury lamp, for example). In alternativeembodiments, source 112 can also be remote from lithographic projectionapparatus 100. In this case, radiation beam 122 would be directed intoapparatus 100 (e.g., with the aid of suitable directing mirrors). Thislatter scenario is often the case when source 112 is an excimer laser.It is to be appreciated that both of these scenarios are contemplatedwithin the scope of the present invention.

Beam 110 subsequently intercepts the array of individually controllableelements 104 after being directed using beam splitter 118. Having beenreflected by the array of individually controllable elements 104, beam110 passes through projection system 108, which focuses beam 110 onto atarget portion 120 of the substrate 114.

With the aid of positioning device 116 (and optionally interferometricmeasuring device 134 on a base plate 136 that receives interferometricbeams 138 via beam splitter 140), substrate table 6 can be movedaccurately, so as to position different target portions 120 in the pathof beam 110. Where used, the positioning device for the array ofindividually controllable elements 104 can be used to accurately correctthe position of the array of individually controllable elements 104 withrespect to the path of beam 110, e.g., during a scan. In general,movement of object table 106 is realized with the aid of a long-strokemodule (course positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. A similarsystem can also be used to position the array of individuallycontrollable elements 104. It will be appreciated that beam 110 canalternatively/additionally be moveable, while object table 106 and/orthe array of individually controllable elements 104 can have a fixedposition to provide the required relative movement.

In an alternative configuration of the embodiment, substrate table 106can be fixed, with substrate 114 being moveable over substrate table106. Where this is done, substrate table 106 is provided with amultitude of openings on a flat uppermost surface, gas being fed throughthe openings to provide a gas cushion which is capable of supportingsubstrate 114. This is conventionally referred to as an air bearingarrangement. Substrate 114 is moved over substrate table 106 using oneor more actuators (not shown), which are capable of accuratelypositioning substrate 114 with respect to the path of beam 110.Alternatively, substrate 114 can be moved over substrate table 106 byselectively starting and stopping the passage of gas through theopenings.

Although lithography apparatus 100 according to the invention is hereindescribed as being for exposing a resist on a substrate, it will beappreciated that the invention is not limited to this use and apparatus100 can be used to project a patterned beam 110 for use in resistlesslithography.

FIG. 2 schematically depicts a lithographic apparatus 200 in accordancewith another embodiment of the present invention. The apparatuscomprises an illumination system IL, a patterning device PD, a substratetable WT, and a projection system PS. The illumination system(illuminator) IL is configured to condition a radiation beam B (e.g., UVradiation).

The substrate table WT is constructed to support a substrate (e.g., aresist-coated substrate) W and connected to a positioner PW configuredto accurately position the substrate in accordance with certainparameters.

The projection system (e.g., a refractive projection lens system) PS isconfigured to project the beam of radiation modulated by the array ofindividually controllable elements onto a target portion C (e.g.,comprising one or more dies) of the substrate W. The term “projectionsystem” used herein should be broadly interpreted as encompassing anytype of projection system, including refractive, reflective,catadioptric, magnetic, electromagnetic and electrostatic opticalsystems, or any combination thereof, as appropriate for the exposureradiation being used, or for other factors such as the use of animmersion liquid or the use of a vacuum. Any use of the term “projectionlens” herein can be considered as synonymous with the more general term“projection system.”

The projection system PS may include dynamic elements, such as asynchronous scanning mirror SSM as described below. The synchronousscanning mirror SSM can require a frequency signal F from the radiationsource SO and scan velocity signal SV from the substrate table WT tofunction, i.e., to control a resonant frequency of the synchronousscanning mirror SSM.

The illumination system can include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device PD (e.g., a reticle or mask or an array ofindividually controllable elements) modulates the beam. In general, theposition of the array of individually controllable elements will befixed relative to the projection system PS. However, it can instead beconnected to a positioner configured to accurately position the array ofindividually controllable elements in accordance with certainparameters.

The term “patterning device” or “contrast device” used herein should bebroadly interpreted as referring to any device that can be used tomodulate the cross-section of a radiation beam, such as to create apattern in a target portion of the substrate. The devices can be eitherstatic patterning devices (e.g., masks or reticles) or dynamic (e.g.,arrays of programmable elements) patterning devices. For brevity, mostof the description will be in terms of a dynamic patterning device,however it is to be appreciated that a static pattern device can also beused without departing from the scope of the present invention.

It should be noted that the pattern imparted to the radiation beam maynot exactly correspond to the desired pattern in the target portion ofthe substrate, for example if the pattern includes phase-shiftingfeatures or so called assist features. Similarly, the pattern eventuallygenerated on the substrate may not correspond to the pattern formed atany one instant on the array of individually controllable elements. Thiscan be the case in an arrangement in which the eventual pattern formedon each part of the substrate is built up over a given period of time ora given number of exposures during which the pattern on the array ofindividually controllable elements and/or the relative position of thesubstrate changes.

Generally, the pattern created on the target portion of the substratewill correspond to a particular functional layer in a device beingcreated in the target portion, such as an integrated circuit or a flatpanel display (e.g., a color filter layer in a flat panel display or athin film transistor layer in a flat panel display). Examples of suchpatterning devices include reticles, programmable mirror arrays, laserdiode arrays, light emitting diode arrays, grating light valves, and LCDarrays.

Patterning devices whose pattern is programmable with the aid ofelectronic means (e.g., a computer), such as patterning devicescomprising a plurality of programmable elements (e.g., all the devicesmentioned in the previous sentence except for the reticle), arecollectively referred to herein as “contrast devices.” The patterningdevice comprises at least 10, at least 100, at least 1,000, at least10,000, at least 100,000, at least 1,000,000, or at least 10,000,000programmable elements.

A programmable mirror array can comprise a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that addressed areas of thereflective surface reflect incident light as diffracted light, whereasunaddressed areas reflect incident light as undiffracted light. Using anappropriate spatial filter, the undiffracted light can be filtered outof the reflected beam, leaving only the diffracted light to reach thesubstrate. In this manner, the beam becomes patterned according to theaddressing pattern of the matrix-addressable surface.

It will be appreciated that, as an alternative, the filter can filterout the diffracted light, leaving the undiffracted light to reach thesubstrate.

An array of diffractive optical MEMS devices (micro-electro-mechanicalsystem devices) can also be used in a corresponding manner. In oneexample, a diffractive optical MEMS device is composed of a plurality ofreflective ribbons that can be deformed relative to one another to forma grating that reflects incident light as diffracted light.

A further alternative example of a programmable mirror array employs amatrix arrangement of tiny mirrors, each of which can be individuallytilted about an axis by applying a suitable localized electric field, orby employing piezoelectric actuation means. Once again, the mirrors arematrix-addressable, such that addressed mirrors reflect an incomingradiation beam in a different direction than unaddressed mirrors; inthis manner, the reflected beam can be patterned according to theaddressing pattern of the matrix-addressable mirrors. The requiredmatrix addressing can be performed using suitable electronic means.

Another example PD is a programmable LCD array.

The lithographic apparatus can comprise one or more contrast devices.For example, it can have a plurality of arrays of individuallycontrollable elements, each controlled independently of each other. Insuch an arrangement, some or all of the arrays of individuallycontrollable elements can have at least one of a common illuminationsystem (or part of an illumination system), a common support structurefor the arrays of individually controllable elements, and/or a commonprojection system (or part of the projection system).

In one example, such as the embodiment depicted in FIG. 1, the substrate114 has a substantially circular shape, optionally with a notch and/or aflattened edge along part of its perimeter. In another example, thesubstrate has a polygonal shape, e.g., a rectangular shape.

Examples where the substrate has a substantially circular shape includeexamples where the substrate has a diameter of at least 25 mm, at least50 mm, at least 75 mm, at least 100 mm, at least 125 mm, at least 150mm, at least 175 mm, at least 200 mm, at least 250 mm, or at least 300mm. Alternatively, the substrate has a diameter of at most 500 mm, atmost 400 mm, at most 350 mm, at most 300 mm, at most 250 mm, at most 200mm, at most 150 mm, at most 100 mm, or at most 75 mm.

Examples where the substrate is polygonal, e.g., rectangular, includeexamples where at least one side, at least 2 sides or at least 3 sides,of the substrate has a length of at least 5 cm, at least 25 cm, at least50 cm, at least 100 cm, at least 150 cm, at least 200 cm, or at least250 cm.

At least one side of the substrate has a length of at most 1000 cm, atmost 750 cm, at most 500 cm, at most 350 cm, at most 250 cm, at most 150cm, or at most 75 cm.

In one example, the substrate 114 is a wafer, for instance asemiconductor wafer. The wafer material can be selected from the groupconsisting of Si, SiGe, SiGeC, SiC, Ge, GaAs, InP, and InAs. The wafermay be: a III/V compound semiconductor wafer, a silicon wafer, a ceramicsubstrate, a glass substrate, or a plastic substrate. The substrate maybe transparent (for the naked human eye), colored, or absent a color.

The thickness of the substrate can vary and, to an extent, can depend onthe substrate material and/or the substrate dimensions. The thicknesscan be at least 50 μm, at least 100 μm, at least 200 μm, at least 300μm, at least 400 μm, at least 500 μm, or at least 600 μm. Alternatively,the thickness of the substrate may be at most 5000 μm, at most 3500 μm,at most 2500 μm, at most 1750 μm, at most 1250 μm, at most 1000 μm, atmost 800 μm, at most 600 μm, at most 500 μm, at most 400 μm, or at most300 μm.

The substrate referred to herein can be processed, before or afterexposure, in for example a track (a tool that typically applies a layerof resist to a substrate and develops the exposed resist), a metrologytool, and/or an inspection tool. In one example, a resist layer isprovided on the substrate.

Referring to FIG. 2, the projection system can image the pattern on thearray of individually controllable elements, such that the pattern iscoherently formed on the substrate. Alternatively, the projection systemcan image secondary sources for which the elements of the array ofindividually controllable elements act as shutters. In this respect, theprojection system can comprise an array of focusing elements such as amicro lens array (known as an MLA) or a Fresnel lens array to form thesecondary sources and to image spots onto the substrate. The array offocusing elements (e.g., MLA) comprises at least 10 focus elements, atleast 100 focus elements, at least 1,000 focus elements, at least 10,000focus elements, at least 100,000 focus elements, or at least 1,000,000focus elements.

The number of individually controllable elements in the patterningdevice is equal to or greater than the number of focusing elements inthe array of focusing elements. One or more (e.g., 1,000 or more, themajority, or each) of the focusing elements in the array of focusingelements can be optically associated with one or more of theindividually controllable elements in the array of individuallycontrollable elements, with 2 or more, 3 or more, 5 or more, 10 or more,20 or more, 25 or more, 35 or more, or 50 or more of the individuallycontrollable elements in the array of individually controllableelements.

The MLA may be movable (e.g., with the use of one or more actuators) atleast in the direction to and away from the substrate. Being able tomove the MLA to and away from the substrate allows, e.g., for focusadjustment without having to move the substrate.

As herein depicted in FIGS. 1 and 2, the apparatus is of a reflectivetype (e.g., employing a reflective array of individually controllableelements). Alternatively, the apparatus can be of a transmission type(e.g., employing a transmission array of individually controllableelements).

The lithographic apparatus can be of a type having two (dual stage) ormore substrate tables. In such “multiple stage” machines, the additionaltables can be used in parallel, or preparatory steps can be carried outon one or more tables while one or more other tables are being used forexposure.

The lithographic apparatus can also be of a type wherein at least aportion of the substrate can be covered by an “immersion liquid” havinga relatively high refractive index, e.g., water, so as to fill a spacebetween the projection system and the substrate. An immersion liquid canalso be applied to other spaces in the lithographic apparatus, forexample, between the patterning device and the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems. The term “immersion” as usedherein does not mean that a structure, such as a substrate, must besubmerged in liquid, but rather only means that liquid is locatedbetween the projection system and the substrate during exposure.

Referring again to FIG. 2, the illuminator IL receives a radiation beamfrom a radiation source SO. The radiation source provides radiationhaving a wavelength of at least 5 nm, at least 10 nm, at least 11-13 nm,at least 50 nm, at least 100 nm, at least 150 nm, at least 175 nm, atleast 200 nm, at least 250 nm, at least 275 nm, at least 300 nm, atleast 325 nm, at least 350 nm, or at least 360 nm. Alternatively, theradiation provided by radiation source SO has a wavelength of at most450 nm, at most 425 nm, at most 375 nm, at most 360 nm, at most 325 nm,at most 275 nm, at most 250 nm, at most 225 nm, at most 200 nm, or atmost 175 nm. The radiation may have a wavelength including 436 nm, 405nm, 365 nm, 355 nm, 248nm, 193 nm, 157 nm, and/or 126 nm.

The source and the lithographic apparatus can be separate entities, forexample when the source is an excimer laser. In such cases, the sourceis not considered to form part of the lithographic apparatus and theradiation beam is passed from the source SO to the illuminator IL withthe aid of a beam delivery system BD comprising, for example, suitabledirecting mirrors and/or a beam expander. In other cases the source canbe an integral part of the lithographic apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, can be referred to as aradiation system.

The illuminator IL, can comprise an adjuster AD for adjusting theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL can comprise various other components, such as anintegrator IN and a condenser CO. The illuminator can be used tocondition the radiation beam to have a desired uniformity and intensitydistribution in its cross-section. The illuminator IL, or an additionalcomponent associated with it, can also be arranged to divide theradiation beam into a plurality of sub-beams that can, for example, eachbe associated with one or a plurality of the individually controllableelements of the array of individually controllable elements. Atwo-dimensional diffraction grating can, for example, be used to dividethe radiation beam into sub-beams. In the present description, the terms“beam of radiation” and “radiation beam” encompass, but are not limitedto, the situation in which the beam is comprised of a plurality of suchsub-beams of radiation.

The radiation beam B is incident on the patterning device PD (e.g., anarray of individually controllable elements) and is modulated by thepatterning device. Having been reflected by the patterning device PD,the radiation beam B passes through the projection system PS, whichfocuses the beam onto a target portion C of the substrate W. With theaid of the positioner PW and position sensor IF2 (e.g., aninterferometric device, linear encoder, capacitive sensor, or the like),the substrate table WT can be moved accurately, e.g., so as to positiondifferent target portions C in the path of the radiation beam B. Whereused, the positioning means for the array of individually controllableelements can be used to correct accurately the position of thepatterning device PD with respect to the path of the beam B, e.g.,during a scan.

In one example, movement of the substrate table WT is realized with theaid of a long-stroke module (course positioning) and a short-strokemodule (fine positioning), which are not explicitly depicted in FIG. 2.In another example, a short stroke stage may not be present. A similarsystem can also be used to position the array of individuallycontrollable elements. It will be appreciated that the beam B canalternatively/additionally be moveable, while the object table and/orthe array of individually controllable elements can have a fixedposition to provide the required relative movement. Such an arrangementcan assist in limiting the size of the apparatus. As a furtheralternative, which can, e.g., be applicable in the manufacture of flatpanel displays, the position of the substrate table WT and theprojection system PS can be fixed and the substrate W can be arranged tobe moved relative to the substrate table WT. For example, the substratetable WT can be provided with a system for scanning the substrate Wacross it at a substantially constant velocity.

As shown in FIG. 2, the beam of radiation B can be directed to thepatterning device PD by means of a beam splitter BS configured such thatthe radiation is initially reflected by the beam splitter and directedto the patterning device PD. It should be realized that the beam ofradiation B can also be directed at the patterning device without theuse of a beam splitter. The beam of radiation can be directed at thepatterning device at an angle between 0 and 90°, between 5 and 85°,between 15 and 75°, between 25 and 65°, or between 35 and 55° (theembodiment shown in FIG. 2 is at a 90° angle). The patterning device PDmodulates the beam of radiation B and reflects it back to the beamsplitter BS which transmits the modulated beam to the projection systemPS. It will be appreciated, however, that alternative arrangements canbe used to direct the beam of radiation B to the patterning device PDand subsequently to the projection system PS. In particular, anarrangement such as is shown in FIG. 2 may not be required if atransmission patterning device is used.

The depicted apparatus of FIGS. 1 and 2 can be used in several modes:

1. In step mode, the array of individually controllable elements and thesubstrate are kept essentially stationary, while an entire patternimparted to the radiation beam is projected onto a target portion C atone go (i.e., a single static exposure). The substrate table WT is thenshifted in the X and/or Y direction so that a different target portion Ccan be exposed. In step mode, the maximum size of the exposure fieldlimits the size of the target portion C imaged in a single staticexposure.

2. In scan mode, the array of individually controllable elements and thesubstrate are scanned synchronously while a pattern imparted to theradiation beam is projected onto a target portion C (i.e., a singledynamic exposure). The velocity and direction of the substrate relativeto the array of individually controllable elements can be determined bythe (de-) magnification and image reversal characteristics of theprojection system PS. In scan mode, the maximum size of the exposurefield limits the width (in the non-scanning direction) of the targetportion in a single dynamic exposure, whereas the length of the scanningmotion determines the height (in the scanning direction) of the targetportion.

3. In pulse mode, the array of individually controllable elements iskept essentially stationary and the entire pattern is projected onto atarget portion C of the substrate W using a pulsed radiation source. Thesubstrate table WT is moved with an essentially constant speed such thatthe beam B is caused to scan a line across the substrate W. The patternon the array of individually controllable elements is updated asrequired between pulses of the radiation system and the pulses are timedsuch that successive target portions C are exposed at the requiredlocations on the substrate W. Consequently, the beam B can scan acrossthe substrate W to expose the complete pattern for a strip of thesubstrate. The process is repeated until the complete substrate W hasbeen exposed line by line.

4. Continuous scan mode is essentially the same as pulse mode exceptthat the substrate W is scanned relative to the modulated beam ofradiation B at a substantially constant speed and the pattern on thearray of individually controllable elements is updated as the beam Bscans across the substrate W and exposes it. A substantially constantradiation source or a pulsed radiation source, synchronized to theupdating of the pattern on the array of individually controllableelements, can be used.

5. In pixel grid imaging mode, which can be performed using thelithographic apparatus of FIG. 2, the pattern formed on substrate W isrealized by subsequent exposure of spots formed by a spot generator thatare directed onto patterning device PD. The exposed spots havesubstantially the same shape. On substrate W the spots are printed insubstantially a grid. In one example, the spot size is larger than apitch of a printed pixel grid, but much smaller than the exposure spotgrid. By varying intensity of the spots printed, a pattern is realized.In between the exposure flashes the intensity distribution over thespots is varied.

Combinations and/or variations on the above described modes of use orentirely different modes of use can also be employed.

In lithography, a pattern is exposed on a layer of resist on thesubstrate. The resist is then developed. Subsequently, additionalprocessing steps are performed on the substrate. The effect of thesesubsequent processing steps on each portion of the substrate depends onthe exposure of the resist. In particular, the processes are tuned suchthat portions of the substrate that receive a radiation dose above agiven dose threshold respond differently to portions of the substratethat receive a radiation dose below the dose threshold. For example, inan etching process, areas of the substrate that receive a radiation doseabove the threshold are protected from etching by a layer of developedresist.

However, in the post-exposure development, the portions of the resistthat receive a radiation dose below the threshold are removed andtherefore those areas are not protected from etching. Accordingly, adesired pattern can be etched. In particular, the individuallycontrollable elements in the patterning device are set such that theradiation that is transmitted to an area on the substrate within apattern feature is at a sufficiently high intensity that the areareceives a dose of radiation above the dose threshold during theexposure. The remaining areas on the substrate receive a radiation dosebelow the dose threshold by setting the corresponding individuallycontrollable elements to provide a zero or significantly lower radiationintensity.

In practice, the radiation dose at the edges of a pattern feature doesnot abruptly change from a given maximum dose to zero dose even if theindividually controllable elements are set to provide the maximumradiation intensity on one side of the feature boundary and the minimumradiation intensity on the other side. Instead, due to diffractiveeffects, the level of the radiation dose drops off across a transitionzone. The position of the boundary of the pattern feature ultimatelyformed by the developed resist is determined by the position at whichthe received dose drops below the radiation dose threshold. The profileof the drop-off of radiation dose across the transition zone, and hencethe precise position of the pattern feature boundary, can be controlledmore precisely by setting the individually controllable elements thatprovide radiation to points on the substrate that are on or near thepattern feature boundary. These can be not only to maximum or minimumintensity levels, but also to intensity levels between the maximum andminimum intensity levels. This is commonly referred to as “grayscaling.”

Grayscaling provides greater control of the position of the patternfeature boundaries than is possible in a lithography system in which theradiation intensity provided to the substrate by a given individuallycontrollable element can only be set to two values (e.g., just a maximumvalue and a minimum value). At least 3, at least 4 radiation intensityvalues, at least 8 radiation intensity values, at least 16 radiationintensity values, at least 32 radiation intensity values, at least 64radiation intensity values, at least 128 radiation intensity values, orat least 256 different radiation intensity values can be projected ontothe substrate.

It should be appreciated that grayscaling can be used for additional oralternative purposes to that described above. For example, theprocessing of the substrate after the exposure can be tuned, such thatthere are more than two potential responses of regions of the substrate,dependent on received radiation dose level. For example, a portion ofthe substrate receiving a radiation dose below a first thresholdresponds in a first manner; a portion of the substrate receiving aradiation dose above the first threshold but below a second thresholdresponds in a second manner; and a portion of the substrate receiving aradiation dose above the second threshold responds in a third manner.Accordingly, grayscaling can be used to provide a radiation dose profileacross the substrate having more than two desired dose levels. Theradiation dose profile can have at least 2 desired dose levels, at least3 desired radiation dose levels, at least 4 desired radiation doselevels, at least 6 desired radiation dose levels or at least 8 desiredradiation dose levels.

It should further be appreciated that the radiation dose profile can becontrolled by methods other than by merely controlling the intensity ofthe radiation received at each point on the substrate, as describedabove. For example, the radiation dose received by each point on thesubstrate can alternatively or additionally be controlled by controllingthe duration of the exposure of the point. As a further example, eachpoint on the substrate can potentially receive radiation in a pluralityof successive exposures. The radiation dose received by each point can,therefore, be alternatively or additionally controlled by exposing thepoint using a selected subset of the plurality of successive exposures.

In order to form the required pattern on the substrate, it is necessaryto set each of the individually controllable elements in the patterningdevice to the requisite state at each stage during the exposure process.Therefore, control signals, representing the requisite states, must betransmitted to each of the individually controllable elements. In oneexample, the lithographic apparatus includes a controller that generatesthe control signals. The pattern to be formed on the substrate can beprovided to the lithographic apparatus in a vector-defined format, suchas GDSII. In order to convert the design information into the controlsignals for each individually controllable element, the controllerincludes one or more data manipulation devices, each configured toperform a processing step on a data stream that represents the pattern.The data manipulation devices can collectively be referred to as the“datapath.”

The data manipulation devices of the datapath can be configured toperform one or more of the following functions: converting vector-baseddesign information into bitmap pattern data; converting bitmap patterndata into a required radiation dose map (e.g., a required radiation doseprofile across the substrate); converting a required radiation dose mapinto required radiation intensity values for each individuallycontrollable element; and converting the required radiation intensityvalues for each individually controllable element into correspondingcontrol signals.

FIG. 2 depicts an arrangement of the apparatus according to the presentinvention that can be used, e.g., in the manufacture of flat paneldisplays. As shown in FIG. 2, the projection system PS includes a beamexpander, which comprises two lenses L1, L2. The first lens L1 isarranged to receive the modulated radiation beam B and focus it throughan aperture in an aperture stop AS. A further lens AL can be located inthe aperture. The radiation beam B then diverges and is focused by thesecond lens L2 (e.g., a field lens).

The projection system PS further comprises an array of lenses MLAarranged to receive the expanded modulated radiation B. Differentportions of the modulated radiation beam B, corresponding to one or moreof the individually controllable elements in the patterning device PD,pass through respective different lenses ML in the array of lenses MLA.Each lens focuses the respective portion of the modulated radiation beamB to a point which lies on the substrate W. In this way an array ofradiation spots S is exposed onto the substrate W. It will beappreciated that, although only eight lenses of the illustrated array oflenses MLA are shown, the array of lenses can comprise many thousands oflenses (the same is true of the array of individually controllableelements used as the patterning device PD).

FIG. 3 illustrates schematically how a pattern on the substrate W isgenerated using the system of FIG. 2. The filled in circles representthe array of spots S projected onto the substrate W by the array oflenses MLA in the projection system PS. The substrate W is movedrelative to the projection system PS in the Y direction as a series ofexposures are exposed on the substrate W. The open circles representspot exposures SE that have previously been exposed on the substrate W.As shown, each spot projected onto the substrate by the array of lenseswithin the projection system PS exposes a row R of spot exposures on thesubstrate W. The complete pattern for the substrate is generated by thesum of all the rows R of spot exposures SE exposed by each of the spotsS. Such an arrangement is commonly referred to as “pixel grid imaging,”discussed above.

It can be seen that the array of radiation spots S is arranged at anangle θ relative to the substrate W (the edges of the substrate lieparallel to the X and Y directions). This is done so that when thesubstrate is moved in the scanning direction (the Y-direction), eachradiation spot will pass over a different area of the substrate, therebyallowing the entire substrate to be covered by the array of radiationspots 15. The angle θ can be at most 20°, at most 10°, at most 5°, atmost 3°, at most 1°, at most 0.5°, at most 0.25°, at most 0.10°, at most0.05°, or at most 0.01°. Alternatively, the angle θ is at least 0.001°.

FIG. 4 shows schematically how an entire flat panel display substrate Wcan be exposed in a single scan using a plurality of optical engines,according to one embodiment of the present invention. In the exampleshown eight arrays SA of radiation spots S are produced by eight opticalengines (not shown), arranged in two rows R1, R2 in a “chess board”configuration, such that the edge of one array of radiation spots (e.g.,spots S in FIG. 3) slightly overlaps (in the scanning direction Y) withthe edge of the adjacent array of radiation spots.

In one example, the optical engines are arranged in at least 3 rows, forinstance 4 rows or 5 rows. In this way, a band of radiation extendsacross the width of the substrate W, allowing exposure of the entiresubstrate to be performed in a single scan. It will be appreciated thatany suitable number of optical engines can be used. In one example, thenumber of optical engines is at least 1, at least 2, at least 4, atleast 8, at least 10, at least 12, at least 14, or at least 17.Alternatively, the number of optical engines is less than 40, less than30 or less than 20.

Each optical engine can comprise a separate illumination system IL,patterning device PD and projection system PS as described above. It isto be appreciated, however, that two or more optical engines can shareat least a part of one or more of the illumination system, patterningdevice and projection system.

FIG. 5 is a general overview of an exemplary illumination system 500arranged in accordance with the present invention. The illuminationsystem 500 includes an illuminator 501 and PO 502. The illuminator 501includes an SLM plane 503, a catadioptric relay 504, and a mask 506. Byway of background, FIGS. 5 and 6 show the end portion of the illuminatorshown in FIGS. 1 and 2.

More specifically, FIG. 5 is a block diagram illustration representingillumination of the SLM plane 503 via the relay 504 within theilluminator 501. Of particular note, the relay 504 does not include abeamsplitter cube, thus enabling an arbitrary polarization state to bepropagated therethrough. The ability to use the arbitrary polarizationstates of radiation is an important feature in high NA imaging systems.

Also, the illumination system 500 of the present invention serves dualpurposes. For example, the illuminator 500 is an imaging system thatimages the mask 506 onto the SLM plane 503. However, the mask 506 alsois intended to eliminate stray light from previous parts (not shown) ofthe illuminator 500. Another role for the mask 506 is to create, forexample, a trapezoidal profile around the border of the SLM plane 503.By way of example, the mask 506 is illuminated telecentrically by aprevious segment of the illuminator 501. More information onillumination systems as referred to herein can be gleaned, for example,from U.S. patent application Ser. No. 11/020,567 entitled LithographicApparatus and Device Manufacturing Method, which is incorporated hereinby reference, in its entirety.

The relay 504 includes a first group of refractive group elements G1creating an intermediate pupil P3. A second group of refractive elementsG2 is included to direct beams to a mirror F1. The mirrors F1 and F2 canbe, for example, fold mirrors. Whether the mirrors are fold mirrors orsome other configuration, the mirrors F1 and F2 are optional and aremerely included in the illuminator 500 for purposes of opticalpackaging.

An optional third group of refractive elements G3 is also shown in FIG.5. Concave mirrors M1, M2, the second fold mirror F2, and the SLM plane503 are provided along with the mask 506. The SLM plane 503 includes aplurality of separate SLMs. However, for purposes of illustration, thedescription of FIG. 5 focuses on SLMs 507 and 508.

The concave mirrors M1 and M2 are beneficial for field curvaturecorrection. Generally, the use of concave mirrors simplifies the overallrelay design in comparison, for example, to an all-refractive design(i.e., fewer numbers of elements and/or elements with asphericsurfaces). Although concave mirrors are preferred, mirrors of othergeometry can be used, such as flat or convex.

An intermediate image of the mask 506 is created after the group G2, asillustrated in FIG. 5. The set of SLMs 507 and 508, within the SLM plane503, is positioned off-axis. This causes the mask 506 to also beoff-axis, enabling all elements of the relay 504 to work with off-axisbeams. This off-axis placement enables the radiation beams to avoidobscuration by the mirrors. This permits optical elements within therelay 504 to be truncated to reduce their dimensions.

During operation, radiation beams travel through the first refractivegroup element G1 and the second refractive group element G2, bothpositioned along a relay optical axis 509. P3 is the focal point of theG1 group element. Since the mask 506 is illuminated telecentrically, thefocal point P3 is in the plane of intermediate pupil. The SLM plane 503includes an active area 510. The mask 506 similarly includes an activearea 512. Both of the active areas 510 and 512 are off center (i.e.,off-axis) of the relay optical axis 509.

After the beams travel through G2, they reflect off the folding mirrorF1 and onto the concave mirror M1 through the optional refractive groupG3. The beams then reflect off the concave mirror M1 and travel againthrough the refractive group G3 and focus at the point P2, which isanother intermediate pupil point. This intermediate pupil point P2 is ageometric focal point of the ellipsoidal mirror M2. Thus, any radiationbeam that passed through the intermediate pupil P2 of the ellipsoidalmirror M2 eventually travels to a second geometrical focal point of theellipsoidal mirror M2, which is entrance pupil P1 within the PO 502.Concave mirror M2 can also have non-ellipsoidal shape.

The projection optics 502 are positioned along a second optical axis 511formed between the projection optics 502 and the relay 504. The relay504 itself consists of two relays with the intermediate image betweenthem. The first relay consists of refractive groups G1 and G2. Thesecond relay is catadioptric (or catoptric) and includes the mirrors M1,M2 and the optional refractive group G3. The first relay (G1, G2) isessential because the intermediate image is usually close to the groupG3 or mirror M1 and physically not accessible in order to place the maskat this image. Thus, one of the functions of the relay G1, G2 is tocreate an accessible plane conjugate to the SLM plane.

FIG. 6 is a block diagram illustration 600 of an exemplary embodiment ofan illuminator 602 arranged in accordance with a second embodiment ofthe present invention. The description of the illumination system 500 ofFIG. 5 applies to the block diagram illustration 600 of FIG. 6, exceptwhere otherwise noted. As noted above, the mirrors F1 and F2 of FIG. 5are optional. FIG. 6, as an example, provides an illustration of anexemplary embodiment or of the present invention in which the mirror F2has been excluded. Further, in the example of FIG. 6, a mirror 604 (M1in FIG. 5) is concave paraboloidal.

It is noted that the refractive group G3 is optional. The refractivegroup G3 helps to correct aberrations, for example, pupil coma. It cancontain one or more refractive elements or may be absent. In the exampleof FIG. 6, the group G3 consists of one meniscus.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections can set forth one or more,but not all, exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

1. An illuminator, comprising: a mask positioned along a first opticalaxis; first and second refractive groupings positioned along the firstoptical axis in cooperative arrangement with the mask; first and secondreflecting devices for reflecting a first image output from the firstand second refractive groupings; and a spatial light modulator (SLM)positioned along the first optical axis in cooperative arrangement withthe first and second reflecting devices; wherein active areas of themask and the SLM are positioned off-axis from the first optical axis;wherein the first and second reflecting devices are arranged to reflecta second image output from the SLM along a second optical axis that isorthogonal to the first optical axis; wherein the second reflectingdevice is an ellipsoidal mirror.